a(n) = (15^n - (-1)^n)/16.

Terms

a(0) =0a(1) =1a(2) =14a(3) =211a(4) =3164a(5) =47461a(6) =711914a(7) =10678711a(8) =160180664a(9) =2402709961a(10) =36040649414a(11) =540609741211